Post-acute care is one of the fastest growing areas of US healthcare spending. About 40% of older Americans discharged from a hospital utilize post-acute care, and spending in this area has more than doubled in the past decade. To hold providers responsible for longitudinal care, Medicare is testing accountable care organizations (ACOs) in two pilot programs: the Medicare Shared Savings Program (MSSP) and the Pioneer ACO program. The impact of these efforts on both Medicare spending and patient outcomes remains uncertain, however. To the extent that post-acute care (PAC) spending is reduced, understanding whether lower costs occur through reductions in the quantity or intensity of post-acute care, or both, will be important. Furthermore, ACOs vary widely in their organizational structure and risk models (e.g., stronger vs. weaker PAC engagement; and more vs. less risk-sharing). The magnitude of any changes in PAC utilization will plausibly be modified by ACO heterogeneity. Apart from the effects of ACOs on post-acute care utilization and spending, there is also considerable uncertainty about how any changes in post-acute care will impact key clinical outcomes such as mortality. Therefore, we propose to use national Medicare data to carry out three aims: 1) To evaluate the impact of ACOs on post-acute care spending and utilization; 2) To describe how ACO heterogeneity modifies ACOs? effects on post-acute care spending; and 3) To evaluate the impact of changes in post- acute care spending and utilization on patient outcomes.